Chemical injection valves are used to inject chemical fluids such as corrosion inhibitors, solvents, and other chemicals into the produced fluid in the bore of a tubing string of a well. The chemical fluids inhibit and alleviate corrosion of the tubing string and crystallization and subsequent deposition of paraffins, sulfates, and the like from the production fluid. Commercially available chemical injection valves, such as that shown in FIG. 1, typically use a spring operated ball-and-seat type of valve closure arrangement. In such a valve, chemical fluid is supplied to the valve from a surface source, and once inside the valve, the pressure of the chemical fluid urges the valve ball away from the seat. However, a spring exerts an opposing force on a valve follower which urges the valve ball to the seat. Therefore, to open the valve, the pressure of the chemical fluid must be greater than the opposing spring force plus the pressure in the tubing string bore. And to close the valve, the pressure of the chemical fluid must be less than the opposing spring force.
The problem with this type of chemical injection valve is that the valve ball and seat are constantly in the flow path of the chemical fluid and are therefore subject to the corresponding negative effects of the flow such as scale build up, deposits, and flow cutting. As a result of these negative effects, the valve ball may not seal tightly against the seat, and if the pressure in the bore of the tubing string is less than the pressure in the supply conduit, injection of chemical fluid into the tubing string bore will continue until the pressure equalizes across the valve. In addition, a surge of pressure in the tubing string bore may force production fluid through the valve and into the supply conduit.